This invention relates to capacitive measurement circuits and systems for measuring the mutual capacitance of a sensing capacitor and a target, and in particular to capacitive measurement circuits and systems where the sensing capacitor has one terminal that is grounded.
Capacitive based sensing and measurement systems measure the mutual electrical capacitance between a sensing capacitor and a target. Capacitive sensors are able to directly sense conditions that directly affect the value of the mutual capacitance between the sensor capacitor and the target. For example, the proximity or distance between a metal object and a capacitor electrode can be directly determined by measuring the mutual capacitance therebetween. In addition, the change in mutual capacitance can be used to determine the rate at which the metal object is moving toward or away from the sensor. The proximity of a non-conductive target can be directly sensed as the value of the capacitance of the sensor capacitor changes due to the proximity, or distance between, the non-conductive target changing the dielectric constant of the sensor capacitor over air. These sensors can be used, for example, in clearance measurement systems for sensing the proximity of a target to a sensor capacitor, such as fast moving turbine blades to a wall, wherein the capacitive sensor is contained flush with the wall surface. Additionally, the thickness of a known dielectric material or the dielectric constant of a material of a known thickness may be directly measured by capacitive sensors.
Capacitance sensors may measure other phenomena indirectly where the phenomenon to be sensed or measured is converted into a motion or changes a dielectric constant between the sensor capacitor and the target that may be directly measured capacitively. For example, the target may be a pressure transducer that includes a diaphragm, having stable deflection properties, that is deflected toward or away from the sensing capacitor by a change in pressure. Fluid flow may also be measured where the flow is converted into a pressure or a displacement that can be measured as described above. Similarly, temperature may be determined if the target moves toward or away from the sensing capacitor due to changes in temperature.
Many prior art systems used a non-grounded sensing capacitor. A non-grounded sensing capacitor is one that has both leads accessible to the user for measuring the desired quantity. One advantage of using non-grounded sensor capacitors is that many circuits exist for measuring the value of the capacitance when both leads are available. However, one lead of the capacitor must be grounded in some situations, for example, situations in which the target is electrically grounded. Such a target can include rotating shafts and jet engine turbine blades wherein the distance or proximity to the target is measured. This is the situation in many distance-measuring situations since the target, which is one lead of the capacitor, is usually not isolated from ground. However, the ease of measuring capacitance is not as straightforward when one lead of the capacitor is grounded.
Capacitance-based clearance measurement systems generally are either static or dynamic in operation. Dynamic measurement systems typically do not measure the capacitance between the target and the sensing capacitor directly. Rather dynamic systems measure the change in capacitance between the target and the sensing capacitor. These prior art systems typically apply a constant voltage to the sensing capacitor and measure the change in capacitance due to the motion of the target towards and away from the sensing capacitor. Accordingly, a dynamic system is unable to provide an indication of the clearance of a target from the sensor if the value of the capacitance is constant and therefore is able to provide measurements only if the capacitance is continuously changing.
Static systems typically measure the capacitance between the sensing capacitor and the target by applying a time-varying voltage, such as a sinusoidal voltage, to the sensing capacitor and measuring the resultant current. Conversely, the capacitance between the sensing capacitor and the target may be determined by applying a time-varying current, such as a sinusoidal current, to the sensing capacitor and measuring the resultant voltage. Other prior art techniques determine the capacitance between the sensing capacitor and the target by measuring the resonant frequency of the sensing capacitor and an inductor of a known value.
The desired measurement quantity also further complicates the use of grounded sensor capacitors. For example, the capacitance, C, between a circular plate of area A and a grounded target, separated by air of dielectric constant ∈, is related to the distance, d, between the plate and the target (neglecting fringing) by the equation,
C=∈A/dxe2x80x83xe2x80x83(1) 
Accordingly, the distance between the sensor and the target is inversely related to capacitance, i.e., as expressed in equation (1), the relationship between C and d is non-linear. If the effects of fringing of the electric field are also included, then the relationship is even more complex. In this case in which an inverse relationship between the measured quantity and the capacitance exists it may be advantageous to measure the reciprocal of the capacitance. However, this severely restricts the sensor circuit options available.
Static systems that drive the sensing capacitor with a time varying signal often use a specially designed transformer. These transformers can be expensive and be large enough to affect, detrimentally, the package size of the sensor. In addition, the accuracy of the sensing circuit is dependent upon properly calibrating the circuit for the particular characteristics of the transformer used to drive the circuit. As the transformer characteristics change, the sensing circuit will become less accurate unless the calibration is performed again.
Static systems may also use a diode bridge circuit to develop a rectified analog output signal that results from the drive signal applied to the sensing capacitor. The accuracy of the capacitance measurement system is dependent upon proper calibration for the specific diodes used within the bridge circuitry. These diode characteristics may change over time and are likely to change with respect to temperature and to temperature differences between the sensor and the individual diodes. In addition, the diodes include a parasitic junction capacitance that may be temperature dependent as well. Thus, calibrating the capacitance measuring system for use with the diode bridge is a complex matter that may need to be repeated over multiple temperatures and over time. This complex calibration procedure will require more processing power and memory storage than simpler techniques.
Often distance measuring transducers or pressure measuring transducers are parts of larger systems, which also employ other sensors and actuators. For example, both types of transducers, distance and pressure, are used in automotive control systems, such as engine control computers and anti-lock braking systems. Similarly, both types of transducers are used in semiconductor fabrication equipment. The information from a distance or pressure measurement transducer is often provided to a digital system, such as a microprocessor or computer for processing. Accordingly, an analog-to-digital converter (A/D) must be added to the system to convert the output signal into a digital format further increasing the complexity, cost, and parts count of the system. Furthermore, the use of A/D""s often degrades the performance of the transducer by introducing conversion errors and time delays. Therefore, what is needed is a circuit and system for accurately measuring the capacitance of a grounded sensor capacitor that is low cost, light weight, physically small, and that provides for a linear relationship between the output signal and the value of the sensed capacitance and provides a digital output without requiring the use of analog-to-digital converters (A/D""s).
A circuit and system for sensing and measuring the mutual capacitance between a sensor capacitor having one grounded lead and a target and providing a direct digital output of the measured capacitance is disclosed. The circuit and system includes a relaxation oscillator coupled to a sensor capacitor and a fixed resistor. The fixed resistor and the sensor capacitor in conjunction with the relaxation oscillator provide a time varying output signal that has a period that is proportional to the mutual capacitance of the sensor capacitor and a target and resistance of the fixed resistor. In addition, the frequency of the output signal is inversely proportional to the mutual capacitance of the sensor capacitor and target. The circuit and system can also include circuitry to compensate for the input capacitance of one or more amplifiers used in the relaxation oscillator. The circuit and system can also include circuitry to effectively increase the resistance of the fixed resistor by a predetermined constant. This allows a smaller resistance value for the fixed resistor to be used with the concomitant reduction in the size of the fixed resistor, which reduces the parasitic capacitance of the fixed resistor. A guard electrode can be formed coaxially surrounding the fixed resistor and coupled to an input amplifier in the circuit to further reduce the parallel parasitic capacitance of the fixed resistor. An interval timer can be coupled to the output of the relaxation oscillator to provide an accurate measurement of the period of the output signal. This value, or the frequency of the output signal, may be used by a calculation module to accurately determine the value of the mutual capacitance, or the reciprocal of the mutual capacitance, and the value of the measured variable. In addition, the system may include predetermined calibration signals that are used as correction values.
In one embodiment, the circuit and system includes a first amplifier having an input and an output, the input connected to the first terminal of the sensing capacitor. The amplifier provides an output signal that is proportional to the input signal that is used to drive a Schmitt trigger, the Schmitt trigger having first and second threshold values defining a hysteresis input to the Schmitt trigger. The Schmitt trigger is configured and arranged to switch an output signal between the first and second output values as a function of the input signal and the first and second threshold values. In particular, the output of the Schmitt trigger is the first output level when the input exceeds the second threshold level, in a positive sense. Similarly, the output of the Schmitt trigger is the second output level when the input exceeds the first threshold level in a negative sense. The Schmitt trigger maintains the previous output whenever the input is between the two threshold levels. The circuit and system further includes a fixed resistor connected in series between the output of the Schmitt trigger and the first terminal of the sensing capacitor. The output signal of the Schmitt trigger has first and second values, wherein the output of the Schmitt trigger is the first value for the first relaxation time and the output of the Schmitt trigger is the second value for a second relaxation time. The sum of the first relaxation time and the second relaxation time is proportional to the capacitance of the sensor capacitor. The first and second relaxation times of the output signal are a function of the capacitance of the sensor capacitor and the resistance of the fixed resistor.
Other forms, features and aspects of the above-described methods and system are described in the detailed description that follows.